Prolonged inhalation of nitrous oxide for four or more days has been shown to lead to marrow aplasia. There are scattered reports of decrease in peripheral white counts in leukemic patients, however there have not been any systematic clinical trials. This protocol is a Phase I evaluation of nitrous oxide in the treatment of adults no longer responsive to conventional anti-leukemic therapy. The objectives of this study are to determine the role of nitrous oxide in suppressing myelopoiesis and to determine the most efficacious way to administer this therapeutic modality. Five patients have thus far been entered onto this study receiving 6 separate treatments which will be evaluated. There were no effects in peripheral white counts or bone marrow cellularity in any patient. Future plans are to use continuous exposures of nitrous oxide for up to 96 hours.